I believe the two pregnant peices you are referring to are the PRE-CATS (pre-catalytic converters.) My 380SL has pre-cats and look exactly as shown in your picture. They are built-into the pipes connecting to the exhaust manifold, one on each side. From what you describe, there is loose material inside one of the pre-cats. the pre-cats have material inside of them to perform as part of the emissions system/catalytic system. In that case, I believe what you are looking at is a replacement of that side of the pre-cat. Other possible explanations include

1) the exhaust piple going to the exhaust manifold is loose. Have you verified that both bolts are tight? Tried moving it with your hands and looking down from the top of the engine when the pipe is cold (for safety)?

2) I doubt this is the case, but I assume you have verified that the exhaust manifold cover is tight on that side of the car, that will also produce a rattle if it's loose enough but from what you describe I doubt it's the cause.